Assessment Comments
Assessment is based on results of monitoring at STORET station 15660005 (Willow Creek at Echo Ave.) from April 2011 through November 2012.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is now presumptively designated for Class A1 (primary contact recreation) uses and for Class B(WW1) aquatic life uses. According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are presumptively designated as Class A1 and Class B(WW1) waters. Thus, for the current assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” (IR 5p) due to violations of Iowa water quality criteria for indicator bacteria. This is a new impairment for this previously unassessed segment. The presumptive Class B(WW1) aquatic life uses are assessed as “fully supporting (IR 2a) based on results of chemical/physical monitoring. The source of data for this assessment is the results of monitoring at STORET station 15660005 (Willow Creek at Echo Ave.) conducted as part of a Clean Water Act Section 319 water quality project from April 2011 through November 2012 [NULL 124].
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as “not supported" (IR 5p) due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric mean of indicator bacteria (E. coli) in the 14 samples collected at station 15660005 during the recreation seasons of 2011 and 2012 were as follows: the geometric mean during the 2011 recreation season was was 188 orgs/100 ml, and the geometric mean during the 2012 recreation season was 409 orgs/100 ml. Seven of the 14 samples (50%) exceeded the Class A1 single sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). In addition, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria indicates impairment of presumptive Class A1 uses at this monitoring site.
Results of chemical/physical monitoring at station 15660005 during the 2011-12 period suggest "full support" of the presumptive Class B(WW1) aquatic life uses. None of the 15 samples analyzed violated the Class B(WW1) criteria for dissolved oxygen, pH, or ammonia. One of the 15 samples (7%) violated the Class B(WW1) criterion for temperature of 32C. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as pH and dissolved oxygen suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations is not significantly greater than 10 percent; thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.
